The Chip Select is a part of using the SPI standard. The MOSI, MISO, and SCK pins can be shared with other chips that also have an SPI interface, but each chip sharing those will need its own Chip Select so the microcontroller can communicate to a specific chip.
This particular chip has two chip selects, one (Chip Select) is for selecting the control interface and the other (Data Select) is for selecting the data interface.
“Data Require” is a typo and should be “Data Request”. That pin is an output that allows the microcontroller to know whether or not the 2048-byte FIFO buffer on the VS1053 can take another 32 bytes, or not.